1. Field of the Invention
This invention relates generally to electro-acoustical drivers and loudspeakers employing electro-acoustical drivers. More particularly, the invention relates to improved configurations for compression drivers.
2. Related Art
An electro-acoustical transducer or driver is utilized as a loudspeaker or as a component in a loudspeaker system to transform electrical signals into acoustical ones. The basic designs and components of various types of drivers are well-known and therefore need not be described in detail. Briefly, a driver receives electrical signals and converts the electrical signals to acoustic signals. The driver typically includes mechanical, electromechanical, and magnetic elements to effect this conversion. For example, the electrical signals may be directed through a circular voice coil that is attached to diaphragm and the voice coil positioned in an air gap with a radially oriented permanent magnetic field provided by a permanent magnet and steel elements of a magnet assembly. Due to the Lorenz force affecting the conductor of current positioned in the permanent magnetic field, the alternating current corresponding to electrical signals conveying audio signals actuates the voice coil to reciprocate back and forth in the air space and, correspondingly, move the diaphragm to which the coil is attached. The voice coil may be attached to a flexible diaphragm that is suspended by one or more supporting elements (e.g., a surround, spider, or the like) such that at least a portion of the diaphragm is permitted to move. Accordingly, the reciprocating voice coil actuates the diaphragm to likewise reciprocate and, consequently, produce acoustic signals that propagate as sound waves through a suitable fluid medium such as air. Pressure differences in the fluid medium associated with these waves are interpreted by a listener as sound. The sound waves may be characterized by their instantaneous spectrum and level.
The driver at its output side may be coupled to an acoustic waveguide, which is a structure that encloses the volume of medium into which sound waves are first received from the driver. The waveguide may be designed to increase the efficiency of the transducer and control the directivity of the propagating sound waves. The waveguide typically includes one open end coupled to the driver, and another open end or mouth downstream from the driver-side end. Sound waves produced by the driver propagate through the waveguide and are dispersed from the mouth to a listening area. The waveguide is often structured as a horn or other flared structure such that the interior defined by the waveguide expands or increases from the driver-side end to the mouth.
Electro-acoustical transducers or drivers may be characterized into two broad categories: direct-radiating types and compression types. A direct-radiating transducer produces sound waves and radiates these sound waves directly into open air (i.e., the environment ambient to the loudspeaker), whereas a compression driver first produces sound waves in a high-pressure enclosed volume, or compression chamber, before radiating the sound waves to the typically much lower-pressure open-air environment. The compression chamber is open to a structure commonly referred as a phasing plug that works as a connector between the compression chamber and the horn. The area of the entrance to the phasing plug is smaller than the area of the diaphragm. This provides increased efficiency compared to a direct-radiating loudspeaker. In a direct-radiating loudspeaker, the output mechanical impedance of the vibrating diaphragm is significantly higher than the radiation impedance that causes “generator” (diaphragm) and “load” (radiation impedance) mismatch. In a compression driver, the loading impedance (entrance to the phasing plug) is significantly higher than the open air radiation impedance. This produces much better matching between “generator” and “load” and increases the efficiency of the transducer. The relative advantages and disadvantages of direct-radiating drivers and compression drivers are well-known to persons skilled in the art. Generally, compression drivers are considered to be superior to direct-radiating drivers for generating high sound-pressure levels. The present disclosure is primarily directed to compression drivers.
As noted, a compression driver utilizes a compression chamber on the output side of the diaphragm to generate relatively higher-pressure sound energy prior to radiating the sound waves from the loudspeaker. Typically, a phasing plug is interposed between the diaphragm and the waveguide or horn portion of the loudspeaker, and is spaced from the diaphragm by a small distance (typically a fraction of a millimeter). Accordingly, the compression chamber is bounded on one side by the diaphragm and on the other side by the phasing plug. The phasing plug is typically perforated in some fashion. That is, the phasing plug includes apertures (i.e., passages or channels) that extend between the compression chamber and the waveguide or horn portion of the loudspeaker to provide acoustic pathways from the compression chamber to the waveguide. The cross-sectional area of the apertures is small in comparison to the effective area of the diaphragm, thereby providing air compression and increased sound pressure in the compression chamber.
The compression driver, characterized by having a phasing plug and a compression chamber, can provide a number of advantages if properly designed. These advantages may include increasing the efficiency with which the mechanical energy associated with the moving diaphragm is converted into acoustic energy. Decreasing the parasitic compliance of air in the compression chamber prevents undesired attenuation of high-frequency acoustic signals. Proper position of apertures in the phasing plug and the lengths of the passages provide delivering sound energy in phase from all parts of the diaphragm, suppressing or canceling high-frequency standing waves in the compression chamber, and reducing or eliminating undesired interfering cancellations in the propagating sound waves.
It is well-recognized by persons skilled in the art that an ongoing need exists for providing improved designs for compression drivers so as to more fully attain their advantages such as high-frequency efficiency, while ameliorating their disadvantages such as detrimental acoustical non-linear effects, irregularity of frequency response, and limited frequency range.